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Altera_Forum
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16 years ago

ALT2GXB problem with XAUI (start of frame alignment)

Hello

Recently I've had my first experience with ALT2GXB megafunction in XAUI mode and I've got a problem.

If I correctly understand 802.3, in double edge 4 lane XGMII start of frame (0xFB) can be placed only at 0 lane. In 64 bit mode (single edge with 8 lanes) start of frame can be placed either at 0 lane or at 4th lane. So when I simulated ALT2GXB megafunction in loopback mode there was a strange issue: frame transmission was correct only if start of frame symbol is aligned with 0 lane. If start of frame symbol is placed at 4th lane, at RX side previous to start of frame IDLE symbols (0x07) treated as data (because RXC lines are zeros). Later I've seen the same result in harware (stratix II gx).

You can see TX and RX waveforms in atachments.

Can anyone explane these transformations of RXC? Is it Altera feature or may be I didn't understand something?

Best regards

1 Reply

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    16 years ago

    The problem was in incorrect interpretation of 64 bit interface. We thought that this interface like XGMII after DDR registers - lanes 0,1,2,3 are on rising (falling) edge and lanes 4,5,6,7 are on falling (rising) edge. We were just confused :) In reality it has another order (RTFM):lanes 0,2,4,6 at clock N and lanes 1,3,5,7 at clock N+1.

    Many thanks to Altera Tech.Support